Method of determining the presence of liquids in pressure vessels



Unite tates 3,048,984 METHOD OF DETERMINING THE PRESENCE OF LIQUIDS IN PRESSURE VESSELS Pritchard P. Ells, Baytown, and Wilburn C. Bollard, Jr.,

Wooster, Tex., assignors to United Rubber & Chemical Company, Baytown, Tex., a corporation of Delaware No Drawing. Filed Mar. 3, 1960, Ser. No. 12,565

4 Claims. (Cl. 62--49) This invention relates to the storage of liquified hydrocarbons in pressure vessels. Particularly, it relates to the removal of such hydrocarbons from pressure vessels and, more particularly, to an improved method of confirming the substantially complete removal thereof from such vessels.

Since various hydrocarbons are gases at normal atmospheric temperatures and pressures, it is necessary to store them in liquified form in pressure vessels. Similarly, such hydrocarbons when transported from place to place are shipped in liquified form. Butadiene-1,3, for instance, may be thus readily transported either by pipe line, if its source is conveniently located to the destination, or by specially designed insulated pressure tank cars, if not. When such tank cars are employed for transporting buta diene, the butadiene is readily removed therefrom by causing it to flow by the application of pressure, through a dip leg and appropriate conduits to a pressure storage vessel. Once the flow from the tank car has ceased, it might be natural to assume that the car is substantially empty. There are occasions, however, when this is not the case. Depending upon the particular car involved and the conditions prevailing at the time of its unloading, a substantial quantity may remain within the tank, although termination of flow therefrom would normally dictate otherwise. To avoid loss to the consumer, therefore, it is highly desirable for the consumer to readily confirm whether or not the tank car is substantially unloaded prior to returning to its source.

Various methods have been proposed for confirming the substantially complete removal of liquid from a pressure tank car. One method that is generally employed involves a pressure differential measurement. By this method the boiling point of any liquid remaining in the tank car is reduced by reducing the pressure to a predetermined low positive gauge reading. A subsequently measured pressure increase occuring because of liquid vaporization at the lower boiling point will indicate the presence of residual liquid within the tank car. While this method is accurate, its accuracy is necessarily dependent upon an extended observation period. The heat necessary to vaporize sufficient liquid to make a pressure diiterential measurement feasible can be absorbed only from sources external to the tank car. Such absorption necessarily requires a period of hours. This presents obvious disadvantages. Both unloading and tank car facilities are undesirably taxed. At the same time, additional labor requirements are created.

There has continued to remain, therefore, a demand for a method of confirming the substantially complete removal of liquid from pressure vessels, particularly insulated pressure tank cars. It is a principal object of this invention to fulfill this demand. It is a further object of this invention to provide a method for determining the presence or absence of a recoverable quantity of liquid in a pressure tank car. Such a method should be accurate and rapid. At the same time, it should be operationally simple and involve no unusual apparatus requirements. It is a still further object of this invention, moreover, to provide a method which overcomes, or at least minimizes, the obvious disadvantages inherent in the prior art method.

In accordance with this invention, these objects have 3,048,984 Patented Aug. 14, 1962 been met in a simple yet unusually effective manner. In general, the improved method of this invention comprises some of the steps above described with respect to a determination based on the presence or substantial absence of a pressure differential within the pressure vessel. The essential feature of this invention, however, is based upon the surprising discovery that an accurate and rapid determination can be made based upon a temperature differential within the vessel.

Although the method of this invention may be employed with any pressure vessel used to store a liquified hydrocarbon gas, it obviously finds particular application with insulated pressure tank cars. Accordingly, for the purposes of this invention, further description of the method will be made with respect to such tank cars. Similarly, while the method is applicable to any liquified normally gaseous hydrocarbon, further description of the method will be directed specifically to butadiene-1,3.

As previously described, in the normal unloading of butadiene-1,3 from a pressure tank car, sufiicient total vapor pressure is applied to the interior of the tank car to cause liquified butadiene to rise through a dip leg for conveyance to a pressure storage tank. The means for conducting this withdrawal constitutes conventional equipment forming an integral part of pressure tank cars, the operation of which is readily understood by those skilled in the art. For the purposes of this invention, therefore, this means need not be further discussed. As withdrawal of the liquid proceeds, a point is reached when substantially no further liquid flows through the dip leg. From this, it would be normal to assume the tank car to be substantially evacuated. Experience has taught, however, that there may, in fact, be residual liquid in the tank to a depth of several inches. The failure to withdraw this liquid may be the result of any of various factors. For example, excessive swirling of liquid about the dip leg may have lowered its level at that point heneath the end of the dip leg. It thus becomes desirable, both from the standpoint of the manufacturer as well as the consumer, to determine accurately whether or not the tank is substantially empty.

To make this determination, according to the method of this invention, the total vapor pressure within the tank car is reduced, correspondingly lowering the temperature and boiling point of any residual liquid within the car. As the pressure is reduced to a predetermined point, residual liquid, if any, absorbs heat from the surrounding bodies, including the vapor, and is vaporized until the temperature of the liquid is below its vaporization point. Although the presence or absence of liquid may be immediately determined upon lowering of the pressure to the predetermined point by taking vapor temperature readings at the top and bottom of the tank car, this may be postponed, if desired, since the reduced vapor pressure and any vapor temperature differential will remain substantially constant for at least a few hours because of the heavy insulation of the tank car. Whenever the determination is made, a temperature differential at the reduced pressure between the top and bottom of the tank car is an accurate indication of the presence of liquid.

The extent to which pressure is reduced within the tank car is not capable of exact definition since it may vary depending upon the hydrocarbon involved. It should be reduced sufiiciently, however, to insure vaporization of a sufi'icient amount of hydrocarbon, if it is present, to result in a temperature differential which will indicate the presence of liquid. How far the pressure should be reduced and what temperature diiferential constitutes a measure of recoverable hydrocarbon may be readily determined through observation by one skilled in the art. In the case of butadiene, for instance, it has been found that a pressure reduction to about 36 p.s.i.g. is adequate for the purposes of this invention. A negative pressure should be avoided so as to eliminate the possible introduction of air and formation of an explosive mixture. At a pressure of 36 p.s.i.g., it has been determined that a 1-2" F. temperature differential may be considered an accurate measure of a substantially liquid-free vessel. As the ditferential increases beyond about 2 F., the presence of correspondingly greater quantities of liquid is indicated. By the method of this invention, temperature differentials ranging from slightly in excess of 2 F. to about 6 F. have resulted in additional recoveries of butadiene ranging from l501000 gallons, obtaned by further carefully executed unloading attempts.

A particular advantage of the method of the present invention is that it requires no unusual equipment nor supervisory control. In fact, the method is readily practiced on conventional pressure tank cars with approved unloading facilities for liquefied petroleum gases. Thus, the temperature or thermometer well existing as standard equipment adequately serves for taking the necessary temperature measurements required by this invention. Obviously, other means such as a thermocouple and registering meter may be employed to make the necessary measurements, although it should be understood that the particular means for measuring a temperature differential forms no part of this invention. Plant personnel ordinarily engaged in the normal unloading of tank cars may be readily instructed in the present method and may easily practice it under normal plant supervision.

Although the present method has been described particularly with respect to insulated tank cars, it is as applicable to other insulated pressure vessels as well as to vessels that are uninsulated. Similarly, the method is applicable to normally gaseous hydrocarbons other than butadiene such as propane, propylene, butane, butylene, pentane, amylene and the like.

The following example further describes the present invention. It is intended as illustrative only and not by way of limitation.

Example A conventional railroad pressure tank car containing liquified butadiene-1,3 under a pressure of 16 p.s.i.g. is unloaded using approved unloading facilities associated with said car. Unloading is continued until vapor substantially free of liquid is withdrawn through the dip leg. At this point, a total of 9,624 gallons of butadiene has been withdrawn from the tank car. The vapor pressure in the tank car is then reduced to 5 p.s.i.g., and temperature readings at the top and at the bottom of the tank car are recorded. Temperature measurements of 83 F. and 78 F. at the top and bottom, respectively, are obtained giving a differential of 5 F. Pressure is again applied to the interior of the tank car in an efiort to recover the additional butadiene. By careful operation, an additional 906 gallons of buadiene, or about 9.5% of the original recovery, is obtained.

When the procedure of the above example is repeated on vessels containing propane, butylene and pentane, similar results are obtained.

We claim:

1. In removing the liquid contents of a pressure vessel containing a liquefied hydrocarbon gas in which said hydrocarbon is removed by applying a pressure thereto causing it to flow from the vessel as a liquid, the method of confirming the apparent substantially complete removal of liquid from said vessel when said flow substantially stops, which comprises: reducing the vapor pressure within said vessel, said reduction being adequate to cause the vaporization of sufiicient liquid hydrocarbon, if it is present, to create a vapor temperature differential between the upper and lower vapor areas of said vessel; and measuring the vapor temperatures in said upper and lower vapor areas of said vessel the degree of temperature difi'erential being a measure of the presence of liquid.

2. A method according to claim 1 in which the hydrocarbon is butadiene-1,3.

3. A method according to claim 2 in which the pressure reduction within said vessel is insufficient to create a negative pressure.

4. A method according to claim 3 in which the pressure within said vessel is reduced to about 3-6 p.s.i.g.

References Cited in the file of this patent UNITED STATES PATENTS 1,590,505 Hanna et al. June 29, 1926 1,962,187 Flock June 12, 1934 2,061,014 Wade Nov. 17, 1936 2,151,935 Petroe Mar. 28, 1939 2,641,929 Eek June 16, 1953 

1. IN REMOVING THE LIQUID CONTENTS OF A PRESSURE VESSEL CONTAINING A LIQUEFIED HYDROCARBON GAS IN WHICH SAID HYDROCARBON IS REMOVED BY APPLYING A PRESSURE THERETO CAUSING IT TO FLOW FROM THE VESSEL AS A LIQUID, THE METHOD OF CONFIRMING THE APPARENT SUBSTANTIALLY COMPLETE REMOVAL OF LIQUID FROM SAID VESSEL WHEN SAID FLOW SUBSTANTIALLY STOPS, WHICH COMPRISES: REDUCING THE VAPOR PRESSURE WITHIN SAID VESSEL, SAID REDUCTION BEING ADEQUATE TO CAUSE THE VAPORIZATION OF SUFFICIENT LIQUID HYDROCARBON, IF IT IS PRESENT, TO CREATE A VAPOR TEMPERATURE DIFFERENTIAL BETWEEN THE UPPER AND LOWER VAPOR AREAS OF SAID VESSEL; AND MEASURING THE VAPOR TEMPERATURES IN SAID UPPER AND LOWER VAPOR AREAS OF SAID VESSEL THE DEGREE OF TEMPERATURE DIFFERENTIAL BEING A MEASURE OF THE PRESENCE OF LIQUID. 